Long Term Evolution (LTE), commonly marketed as 4G LTE, is a standard for wireless communication of high-speed data for mobile phones and data terminals. LTE is based on Global System for Mobile Communications (GSM) and Universal Mobile Telecommunication System (UMTS) technologies that provides higher data rate, lower latency and improved system capacity. In LTE systems, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, referred as evolved NodeBs (eNBs), communicating with a plurality of mobile stations, referred as user equipments (UEs).
In LTE Release-14, the study of Latency and Reduction technique is conducted. Among the conclusions, it was observed that reducing transmission time interval (TTI) and processing time can significantly reduce the user plane Uu latency, and improve TCP throughput. For example, to further reduce file transfer delay, the concept of shorten TTI is introduced to enhance TCP throughput. The initial throughput of a TCP connection is upper bounded by a congestion window size (CWND). The growing speed of the CWND is determined by TCP round trip time (RTT), i.e., the time interval length between when a TCP server sends a TCP segment (packet) and when the TCP server receives a TCP acknowledgement (TCP ACK) corresponding to the transmitted TCP segment (packet). By applying a shorter TTI, we can have a shorter HARQ RTT and a shorter TCP RTT, causing a higher CWND growing speed and thus a larger TCP initial throughput.
Short TTI, however, has more overhead than legacy TTI, caused by Layer 1 overhead for Physical downlink control channel (PDCCH) and Layer 2 overhead for HARQ process and packet segmentation. Due to more overhead, the delay reduction gain from short TTI depends on different scenarios. In general, shorter TTI may bring little or even negative delay reduction gain in unsuitable scenarios including low Uu throughput, large file size, long backhaul delay, large L1/L2 overhead for short TTI, and low TCP slow-start threshold.
As a result, dynamic TTI switching is desired, and a UE can be dynamically (with a subframe to subframe granularity) scheduled with legacy TTI unicast physical downlink shared channel (PDSCH) and/or short TTI unicast PDSCH. However, eNodeB has no necessary information to judge whether the considered scenario is suitable for short TTI configuration. Without decision-making information, eNodeB cannot optimize TTI selection to reduce latency and enhance throughput.
A solution is sought.